Document WO 2012/136345 A2 discloses a system and a method for the visual representation of information on real objects. A projection unit transmits graphical or pictorial information on an object, whereas a dynamic tracker determines and tracks the position and/or the location of the object and/or the projection unit in space using a 3D-sensing system. A control correspondingly adapts the transmission of the piece of information to the current position, and/or location of the object or of the projection unit as determined by the tracker. To determine the pose of the object and to calibrate and/or track the projection unit and/or the object, it is possible to fasten markers at predetermined reference points, the reflected light of which is detected by the tracker. Alternatively, a stripe light scanning process may also be carried out in which the projection unit projects an image which is detected and then triangulated or reconstructed using one or several cameras. Points on the object are scanned in accordance with a predetermined system (scanning), and for the calculation of the pose of the object, an iterative best-fit strategy is used. Depending on the projection technique, stripes or other patterns (video projection) or also merely individual points or lines (laser projector) are here scanned over the entire surface.
Document EP 2 642 246 A2 describes a method by which template patterns can be accurately projected onto a workpiece, for example within the framework of the instruction of a fitter upon assembly of components. In this method, one or more precisely calibrated cameras are used for the direct detection of the position of the spot or pattern projected by a laser projector on the surface of the workpiece. Using photogrammetric analyzing techniques, the position of the projector with respect to the cameras is determined. To this end, the detection of features on the workpiece surface by the laser projector is provided, in particular using a mechanical sensing head. A corrected template pattern is then projected onto the workpiece on the basis of the calculated relative position of the workpiece and the projector.
If, for manufacturing reasons or other reasons, the actual surface of the workpiece does not match with that of the CAD model of the workpiece on which the control of the projection is based, document EP 2 642 247 A2 proposes a method by which the laser projection may be adapted accordingly. To this end, alignment features with an order of priority are defined on the workpiece, with which the projection is then aligned.
In addition to projection systems of this type, there are furthermore pure measurement systems by which the geometrical shape and the position and orientation of an object may be determined by an optical and/or mechanical sensing. Different measuring setups are employed in the industry depending on the size of the object. In case of small object sizes, stripe light scanners are often used, in particular mobile devices having an integrated projector and an integrated camera. For a digitalization of 360°, the object may possibly be placed onto a rotary table. Small objects may also be detected in scan chambers which are equipped with a photogrammetric device composed of several cameras and projectors or of one camera and one projector in connection with a rotating table for the object. The advantage of a chamber consists in the optimum conditions for photogrammetry (dark, no influences from the outside). Medium-sized objects are usually calibrated by using coordinate measuring devices which are typically permanently installed in a specific place. Laser scanners are used for large objects which, if necessary, have to be installed in different places within the framework of measuring to ensure a sufficient detection of the objects from several angles of view.
All these (also the mobile) measuring techniques have in common that they have to be carried out by skilled measuring technicians. Furthermore, the workpieces have to be brought to specific places for measuring in certain methods, which is connected with logistic difficulties and a considerable time delay.
The matching following the calibration within the framework of a desired target-actual comparison usually requires two stages: at first, an initialization is carried out to determine an approximate pose of the workpiece, before a nonlinear optimization is then realized. The initialization requires manual inputs into the used measuring technique software. Depending on the situation, one to three striking features (edges, planes, bores etc.) have to be identified in the on-screen display, for example by use of a mouse. In the subsequent nonlinear optimization, the so-called floating into position, the point cloud is displaced and rotated in small steps with respect to the CAD model, and the respective perpendicular distances from the surface are calculated. This is carried out in an iterative manner until the error sum of squares is minimal. A sufficiently good initialization is required for the optimization, such that the global minimum can be obtained (the algorithm would otherwise end in a local minimum, and the resulting matching would be incorrect).
After matching, a test report is prepared from the protocol of the deviations between the target and the actual state of the object. The test report is evaluated and, if necessary, is used on the spot to take countermeasures in case deviations are too large.
Such a “measuring” test of workpieces is realized only very selectively due to the described complexity. Merely highly critical aspects are checked in job lots, less critical aspects, however, are not checked at all. In the large-scale production, the checks are usually carried out only randomly, for example every 100th piece.
Due to the delayed availability of the test report, high costs and losses of time may arise in either case. During the manual assembly, a standstill can be generated if the further mounting has to be interrupted due to quality difficulties until the test report is available on the basis of which the problems may be solved by a correction or a wrecking. In the series production, rejects are produced within the time period between the appearing of the problem and the availability of the test report. Valuable personal/machine capacities are therefore unnecessarily bound and resources wasted.